The present invention relates to a vortex flow meter utilizing the Karman vortexes created by a bluff body or a vortex generator disposed in a fluid.
As is well known, when a pillar-shaped bluff body is placed in a fluid, the flow of the fluid peels off to both side surfaces of the bluff body and a regular vortex train, that is, Karman vortexes occur alternately to the downstream side of the bluff body. The number of Karman vortex occurrences is proportional to the velocity or quantity of fluid flowing. The quantity of flow can be measured by counting the number of Karman vortexes.
However, in the conventional manner, an ultrasonic wave is used to count the number of Karman vortexes and the frequency modulation or the phase modulation is detected thereby. The accuracy of this method is effected by external noises or the like thus resulting in errors in measurement. Since the ultrasonic wave scatters about, the power of the signal received is considerably diminished. Furthermore the flow of the fluid is effected at a place near the vortex so that the ratio of S/N (signal/noise) is lowered due to the interference with the turbulence of the vortex. It may be another reason for inacurracies in measurement. Furthermore, the measurement of the fluid's flow is badly effected by interference from the reflection of the wave off the wall. surface opposite the transmitted ultrasonic wave. Usually, in order to prevent such an effect, a sound absorbing material like woven cotton or woolen cloth is placed on the inner wall surface of the tube in such a manner that the measuring of the fluid's flow can be effectively done in a purified liquid. It cannot be done effectively in the case of dirty or impure liquid that is mixed with dust, mist, etc.
It is for the above reasons that the present applicant has proposed a method in which no sound absorbing material is needed. This method is one in which another tube is passed through the wall of the fluid-flowing tube so as to intersect with the vortex generator at the downstream side. The variation of the flow of the liquid is due to the pressure variation caused by pressure guiding holes bored at a place near each of the wall surfaces of the tube member. It is detected by ultrasonic wave transmitting and receiving apparatuses which are installed respectively at both ends of the tube.
However, in such a manner as described heretofore, it may be necessary to provide heat-proof type ultrasonic wave transmitting and receiving apparatuses in the case of measuring fluids with high temperatures. It follows then that the cost for manufacturing the device proposed will be highly increased. Furthermore there will exist the inconvenience of removing the main body of the flow meter from the connecting tubes.